The present invention relates generally to vapor deposition equipment and methods for depositing thin films, and more particularly to equipment and methods for vapor deposition of ultra-thin passivation layers on the surfaces of micromechanical devices.
In 1987, Larry J. Hornbeck, a scientist with Texas Instruments Incorporated (TI), invented a remarkable micromechanical device, which he initially called a deformable mirror device, but today is called a digital micromirror device or simply a DMD. The DMD is fabricated on a semiconductor chip and includes an array of hinge-mounted microscopic mirrors, each overlying an addressable memory cell whose binary state determines the ON or OFF position of its micromirror. The DMD chip is the basis for various imaging systems, including TI's amazing Digital Light Processing technology, which is used in digital home TV systems and motion picture projectors for movie theaters.
An early generation hinge-mounted DMD is described in Hornbeck U.S. Pat. No. 5,331,454, which discloses a solution to a sticking problem in which a special passivation layer is deposited on the metal surfaces of the DMD elements that repeatedly contact each other. The Hornbeck '454 patent is hereby incorporated by reference. FIGS. 1a and 1b of the Hornbeck '454 patent, which are reproduced herein with the same figure designations and reference numerals, show one micromirror 12 of a DMD chip in which the micromirror (referred to as a deflection element) is positioned first in its undeflected position (FIG. 1a) and then in its deflected position (FIG. 1b) under the electrostatic influence of an underlying address electrode 10. The micromirror 12 rotates on a hinge 14, which is secured in a support layer 16 disposed above a substrate 20. In the deflected position depicted in FIG. 1b, a corner of the micromirror 12 comes into contact with a landing electrode 18, which stops the micromirror's rotation at a precise angle of deflection from its undeflected position. The micromirror and electrodes of the device consist essentially of aluminum.
Attractive inter-molecular forces, known as Van der Waals forces, tend to cause the contacting surfaces to stick together. These forces gradually increase as the repeated contacting action causes the area of the contacting surfaces to gradually increase. Eventually, the Van der Waals forces exceed the restorative forces, leaving the micromirror 12 stuck in its deflected position. When this occurs, image quality is degraded, requiring replacement of the DMD chip in the imaging system. The term “stiction,” which is short for “static friction,” generally is used to refer to this sticking phenomenon.
The Hornbeck '454 patent explains how the deposition of a passivation layer on the surfaces of the micromirror and the landing electrode helps to prevent the build up of Van der Waals forces and the resulting sticking problem. The preferred passivant for the passivation layer is perfluordecanoic acid (PFDA). FIGS. 3a, 3b, and 3c of the Hornbeck '454 patent are also reproduced herein. FIG. 3a shows the molecular structure of a molecule of PFDA, which is a long-chain aliphatic halogenated polar compound having a COOH group at its polar end 34. Following a plasma surface-activation step, a PFDA deposition step deposits an ultra-thin “monolayer” of PFDA on the activated surfaces, typified schematically in FIG. 3b. The deposited single-molecule thick layer has each molecule oriented with the polar end 34 strongly bonded to the contacting surfaces of the micromirror 36 and the landing electrode 38, as depicted in FIG. 3c, in which the PFDA molecules are shown greatly exaggerated in relative size. The free end of each molecule terminates in a CF3 group that is responsible for low Van der Waals surface forces. The deposited PFDA monolayer effectively eliminates performance-degrading stiction.
Hornbeck and TI gradually brought DMD technology from early generation prototypes to a commercial DMD chip by the mid-1990's. Hornbeck U.S. Pat. No. 5,535,047 describes a later generation DMD structure in which each micromirror is elevated above a supporting yoke. The yoke is hinge-mounted and includes landing tips that contact landing sites of a stationary electrode when the yoke is rotated to a fully deflected position. TI's present commercial DMD chips use such elevated-mirror, hinged-yoke architecture with each micromirror representing one pixel in a very large array of pixels. The Hornbeck '047 patent is hereby incorporated by reference.
FIGS. 2, 6 and 7 of the Hornbeck '047 patent, which are reproduced herein, illustrate one pixel 18 in an exploded perspective view (FIG. 2), and in schematic cross-sections in an undeflected position (FIG. 6) and a deflected position (FIG. 7). The pixel 18 is multi-level structure constructed above a substrate 64 that includes addressable memory cells, such as conventional SRAM cells (not shown), which change their binary states to determine the changing positions of each associated micromirror 30. Each mirror 30 is supported by a post 34 that is mounted on a yoke 32. The yoke 32 rotates on a pair of torsion hinges 40 (FIG. 2). The other end of each hinge 40 is attached to a cap 42, which is supported by a post 44. The position of the yoke 32, and thus also the mirror 30, is determined by voltages applied to address electrodes 26 and 28 and a reset/bias bus 60 on the bottom level, and to address electrodes 50 and 52 supported at the intermediate level by posts 54 and 56. The yoke 32 is shown with cross-hatched portions 74 and 78 in FIG. 2 that are attracted to the respective underlying address electrodes 26 and 28. Similarly, the cross-hatched portions 82 and 84 of the mirror 30 are attracted to the respective underlying address electrodes 50 and 52. The reset/bias bus 60 has extensions that define landing sites 62. The yoke 32 has landing tips 58 that contact respective landing sites 62 when the yoke is deflected to either one of two deflected positions. The contacting action between respective landing tips 58 and landing sites 62 can give rise to stiction forces, which are lessened by the deposition of a PFDA anti-stiction layer.
The Hornbeck '454 patent describes methods for depositing a PFDA monolayer on the aluminum contacting surfaces of the device. For example, a solid source of PFDA is heated to its melting temperature to produce a vapor, which then forms the PFDA monolayer on the exposed aluminum surfaces of the device.
FIG. 4 schematically illustrates a prior art system 100 for depositing PFDA on DMD chips. The system includes a deposition chamber 110, which is a box-like configuration having vertical sidewalls 112 and 114, a bottom wall 116, and a ceiling wall 118 that define a sealed enclosure. A base plate 120, which is suspended by the sidewalls, serves as a support for a shelved cassette 122. The cassette 122 holds multiple wafers 124 that contain DMD chips. It will be appreciated by those skilled in the semiconductor art that such wafers each have a large number of chips that are later separated from the wafer and packaged as individual DMD chips. Although FIG. 4 shows only five wafers 124 held in a stacked arrangement in the cassette 122, it will also be appreciated that a typical cassette can carry many more wafers in practice. The cassette 122 is open on its front and rear sides to allow gas vapor to flow through and react with the surfaces of the wafers 124.
The chamber 110 has a front door (not shown) through which the cassette 122 passes at the beginning of a deposition process. The cassette may be robotically loaded into the chamber 110, as is conventional with deposition equipment used in semiconductor processing. After loading of the cassette 122, the door is closed and sealed so that a partial vacuum can be pulled inside the chamber. A heater (not shown) precisely controls the temperature within the chamber 110. The walls 112, 114, 116, and 118 of the chamber provide a sealed enclosure against the outside atmosphere. A sealed fitting 126 in the ceiling wall 118 provides a connection point for a gas input line 128. Gas flowing in the line 128 enters the chamber 110 through a nozzle 130 retained in the fitting 126. The nozzle 130 defines a gas inlet to the chamber 110. A gas outlet for gas exiting the chamber 110 is provided by a sealed fitting 132, which may be in a back wall (not shown) or in the sidewall 114, where connection is made to an effluent line 134.
The deposition system 100 has a gas input line 136 for receiving N2 gas from a source 140 of dry nitrogen. After the cassette 122 has been loaded into the chamber 110 and the chamber has been sealed, the chamber is purged with nitrogen. This sets the stage for the deposition process. A vacuum pump 142 pulls a partial vacuum in the chamber 110 and draws gas out of the chamber through intermediate devices, which are described below. Nitrogen flows into the chamber from the source 140 through a mass flow controller 144 and a valve 146, which are connected in series to a line 148 that is connected to the input line 128. A second mass flow controller 150 controls nitrogen flow through an alternate path during vapor deposition. Electrically driven solenoid devices (not shown) precisely operate the mass flow controllers 144 and 150. Such equipment is well known. The valve 146 and similar valves in the system 100 are pneumatically operated on/off valves.
A vaporizer 152 is used to heat powdered PFDA to a vapor. To initiate vapor deposition, valve 146 is turned off. Nitrogen gas, which serves as a carrier for the PFDA vapor, is provided to the vaporizer 152 through the mass flow controller 150 and a valve 154. PFDA vapor is carried in the nitrogen gas stream into the chamber 110 from the vaporizer 152 through a valve 156 and a step-motor driven throttle 158, which precisely controls the vapor flow rate. A second step-motor driven throttle 160 connected to the effluent line 134 cooperates with the first throttle 158 to provide uniform vapor flow through the chamber 110. Excess PFDA that does not react in the chamber flows out through effluent line 134, the throttle 160, a valve 162 and into a trap 164, where it solidifies. Nitrogen gas that is essentially free of PFDA flows out of the trap 164 through a valve 166, and then through the vacuum pump 142 to an exhaust line 168, where it leaves the system 100. Pure nitrogen from the source 140 is also supplied to the trap 164 through a valve 170. During cleaning and maintenance, the chamber 110 can be isolated from the vaporizer 152 by turning off the valve 156. The vaporizer 152 can be purged through the trap 164 by opening a connecting valve 172 and passing nitrogen through the vaporizer and the trap.
Despite precise control of the PFDA vapor flow rate through the chamber 110, the system 110 did not provide uniform PFDA deposition on the DMD surfaces of the wafers 124. It was found that small particles of PFDA tended to form in the gas lines as the vapor flowed from the vaporizer 152 to the chamber 110. Such particles would deposit on the mirror surfaces resulting in defective DMD chips. The invention addresses this problem.